Fingerprint sensors that measure the fingerprint pattern using electric field sensing methods have become established. U.S. Pat. Nos. 5,940,526 and 5,963,679, assigned to the assignee of the present application, are examples of this type of fingerprint sensor, and the entire contents of which are incorporated herein by reference. These systems measure the fingerprint pattern by establishing an electric field between the finger and the sensor array, and measuring the spatial fluctuations in field strength at the sensor array caused by the shape of the fingerprint ridge and valley pattern.
In some recent applications, the sensor may desirably capture images of fingerprint patterns from fingers that are farther away from the sensor array than is typical with today's technologies. Unfortunately, as the finger gets farther away from the sensor array (for example when a relatively thick dielectric lies between the sensor array and the finger) the spatial field strength variations that represent the fingerprint pattern become weaker. One way to compensate for this loss of spatial pattern strength is to increase the voltage of the signals that generate the field between the finger and the sensor array. The fingerprint spatial pattern strength increases proportionately.
There may be limitations, however, on how much voltage can be placed on the finger and on the sensor array as well. When the voltages on the finger are too high, certain persons with very sensitive fingers may feel that voltage as a slight tingling. This may be undesirable in a consumer product. On the other hand, when voltages are too high on the sensor array, the sensor readout electronics may not perform adequately, for example, they may saturate and generate unacceptable noise, and may even be damaged.
U.S. Pat. No. 5,940,526 describes a system where a drive voltage is impressed on the finger (through a finger drive electrode) and the sensor array reference is connected to a device ground. The electric field is established between the voltage on the finger and a grounded sensor reference plane in arrangements known as driven finger systems. This system works well for imaging fingers over shorter distances. However, the human body has an inherent capacitance to earth ground, hence when a voltage is impressed on the finger, current flows through the finger and body to that ground. When the voltage on the finger is increased, people with sensitive fingers may feel that ground current as a tingling sensation.
U.S. Pat. No. 5,963,679 describes a sensor where a drive voltage is impressed on a reference electrode positioned beneath the sensing array elements, while the finger is connected to the system ground (through a finger drive electrode), in arrangements known as driven sensor systems. This system may work well for imaging fingers over short distances; and for those systems, it maintains the finger voltage close to ground.
However two circuit related problems may currently limit this implementation. First, the small spatial voltage differences representing the ridge-valley pattern are now riding on top of a relatively large common mode voltage from the nearby reference electrode, making measurement of the small spatial voltage differences difficult. Second, the sensor readout electronics, fabricated with standard economical CMOS devices, may not work properly if the voltages on the sensor array exceed the operating range of those devices.
In other words, the detected signals generated from the sensor array and based upon placement of the user's finger adjacent the sensor array are relatively small compared to the drive signal. Thus, these relatively small detected signals may be increasingly difficult to process along with the relatively high drive signal, limiting measurement resolution of the detected signals, for example. Amplifier and processing stages that read and process the detected signals may add additional noise. Another source of noise may be fixed pattern noise from the sensor array, which also may make it increasingly difficult measure the detected signals.